The present invention relates generally to the formation of dielectric films. More specifically, the invention relates to dielectric materials and films comprising same having a low dielectric constant and enhanced mechanical properties and methods for making same.
There is a continuing desire in the microelectronics industry to increase the circuit density in multilevel integrated circuit devices such as memory and logic chips to improve the operating speed and reduce power consumption. In order to continue to reduce the size of devices on integrated circuits, the requirements for preventing capacitive crosstalk between the different levels of metallization becomes increasingly important. These requirements can be summarized by the expression “RC”, whereby “R” is the resistance of the conductive line and “C” is the capacitance of the insulating dielectric interlayer. Capacitance “C” is inversely proportional to line spacing and proportional to the dielectric constant (k) of the interlayer dielectric (ILD). Such low dielectric materials are desirable for use, for example, as premetal dielectric layers or interlevel dielectric layers.
A number of processes have been used for preparing low dielectric constant films. Chemical vapor deposition (CVD) and spin-on dielectric (SOD) processes are typically used to prepare thin films of insulating layers. Other hybrid processes are also known such as CVD of liquid polymer precursors and transport polymerization CVD. A wide variety of low K materials deposited by these techniques have been generally classified in categories such as purely inorganic materials, ceramic materials, silica-based materials, purely organic materials, or inorganic-organic hybrids. Likewise, a variety of processes have been used for curing these materials to decompose and/or remove volatile components and substantially crosslink the films such as heating, treating the materials with plasmas, electron beams, or UV radiation.
The industry has attempted to produce silica-based materials with lower dielectric constants by incorporating organics or other materials within the silicate lattice. Undoped silica glass (SiO2), referred to herein as “USG”, exhibits a dielectric constant of approximately 4.0. However, the dielectric constant of silica glass can be lowered to a value ranging from 2.7 to 3.5 by incorporating terminal groups such as fluorine or methyl into the silicate lattice. These materials are typically deposited as dense films and integrated within the IC device using process steps similar to those for forming USG films.
An alternative approach to reducing the dielectric constant of a material may be to introduce porosity, i.e., reducing the density of the material. A dielectric film when made porous may exhibit lower dielectric constants compared to a relatively denser film. Porosity has been introduced in low dielectric materials through a variety of different means. For example, porosity may be introduced by decomposing part of the film resulting in a film having pores and a lower density. Additional fabrication steps may be required for producing porous films that ultimately add both time and energy to the fabrication process. Minimizing the time and energy required for fabrication of these films is desirable; thus discovering materials that can be processed easily, or alternative processes that minimize processing time, is highly advantageous.
The dielectric constant (K) of a material generally cannot be reduced without a subsequent reduction in the mechanical properties, i.e., elastic modulus (Young's modulus), hardness, toughness, of the material. Mechanical strength is needed for subsequent processing steps such as etching, CMP (“Chemical Mechanical Planarization”), and depositing additional layers such as diffusion barriers for copper, copper metal (“Cu”), and cap layers on the product. Mechanical integrity, or stiffness, compressive, and shear strengths, may be particularly important to survive CMP. It has been found that the ability to survive CMP may be correlated with the elastic modulus of the material, along with other factors including polishing parameters such as the down force and platen speed. See, for example, Wang et al., “Advanced processing: CMP of CU/low-K and Cu/ultralow-K layers”, Solid State Technology, September, 2001; Lin et al., “Low-k Dielectrics Characterization for Damascene Integration”, International Interconnect Technology Conference, Burlingame, Calif., June, 2001. These mechanical properties are also important in the packaging of the final product. Because of the trade-off in mechanical properties, it may be impractical to use certain porous low dielectric compositions.
Besides mechanical properties, an additional concern in the production of a low dielectric film may be the overall thermal budget for manufacture of the IC device. The method used extensively in the literature for cross-linking a low dielectric film and/or introducing porosity into a film is thermal annealing. In the annealing step, or a curing step, the film is typically heated to decompose and/or remove volatile components and substantially cross-link the film. Unfortunately, due to thermal budget concerns, various components of IC devices such as Cu metal lines can only be subjected to processing temperatures for short time periods before their performance deteriorates due to undesirable diffusion processes. Additional heating and cooling steps also can significantly increase the overall manufacturing time for the device, thereby lowering the throughput.
An alternative to the thermal anneal or curing step is the use of ultraviolet light in combination with an oxygen-containing atmosphere to create pores within the material and lower the dielectric constant. The references, Hozumi, A. et al. “Low Temperature Elimination of Organic Components from Mesostructured Organic-inorganic Composite Films Using Vacuum Ultraviolet Light”, Chem. Mater. 2000 Vol. 12, pp. 3842–47 (“Hozumi I”) and Hozumi, A et al., “Micropatiterned Silica Films with Ordered Nanopores Fabricated through Photocalcination”, NanoLetters 2001 1(8), pp. 395–399 (“Hozumi II”), describe removing a cetyltrimethylammonium chloride (CTAC) pore-former from a tetraethoxysilane (TEOS) film using ultraviolet (“VUV”) light (172 nm) in the presence of oxygen. The reference, Ouyang, M., “Conversion of Some Siloxane Polymers to Silicon Oxide by UV/Ozone Photochemical Processes”, Chem. Mater. 2000,12(6), pp.1591–96, describes using UV light ranging from 185 to 254 nm to generate ozone in situ to oxidize carbon side groups within poly(siloxane) films and form a SiO2 film. The reference, Clark, T., et al., “A New Preparation of UV-Ozone Treatment in the Preparation of Substrate-Supported Mesoporous Thin Films”, Chem. Mater. 2000, 12(12), pp. 3879–3884, describes using UV light with a wavelength below 245.4 nm to produce ozone and atomic oxygen to remove organic species within a TEOS film. These techniques, unfortunately, may damage the resultant film by chemically modifying the bonds that remain within the material.
U.S. Pat. No. 4,603,168 describes cross-linking an alkenyl-organopolysiloxane or organohydrosiloxane film by exposure to UV light or electron beam radiation in the presence of heat. The film further includes a dopant such as a photosensitizer like benzophenone or a platinum catalyst that is present in small concentrations to initiate and catalyze the cross-linking. Likewise, the reference Guo, et al., “Highly Active Visible-light Photocatalysts for Curing Ceramic Precursor”, Chem. Mater. 1998, 10(2), pp. 531–36, describes using a platinum bis(beta-diketonate) catalyst to cross-link oligo(methylsilylene)methylene and tetravinylsinlane using UV/visible light. The presence of metal catalysts and chromophores would be unsuitable in dielectric films.
U.S. Pat. No. 6,284,500 describes using UV light in the from 230 to350 nm wavelength range to photoinitiate cross-linking within an organic polymer film formed by CVD or an organosilsiquoxane formed by spin-on deposition and improve the adhesion and mechanical properties of the film. A thermal annealing step may be used to stabilize the cross-linked film.
U.S. Pat. Nos. 5,970,384 and 6,168,980 describe exposing a PVD or CVD deposited oxide gate layer to UV light in the presence of N2O, NH3, or N2H4 at temperatures between 300 and 700° C. The methods described in both the '384 and '980 patents reduce the C and H impurities within the oxide gate layer and introduce nitrogen near the boundary of the material with the silicon substrate.
Besides UV exposure, a further method to process low dielectric films without effecting the thermal budget of the manufacturing of the device may be by exposure to electron beam (“e-beam”) radiation. The electron beam radiation step may be in addition to, or in lieu, of a thermal cure step. It is believed that the e-beam exposure may improve the mechanical properties of the film by removing most or all of the organic species from the film. For example, U.S. Pat. No. 6,042,994 describes a process wherein a nanoporous dielectric coated substrate is treated with a large area electron beam exposure system. The '994 patent contends based upon FTIR data that the e-beam cure has removed most of the organic species from the film. WO 97/00535 teaches a process for curing a dielectric material such as a spin-on-glass (SOG) having about 10–25% organic groups by exposure to e-beam irradiation. Using FTIR analysis, the WO 97/00535 application reports that there are no longer CH groups attached to the backbone of SOG starting compounds after curing with e-beam radiation.
Accordingly, there is a need in the art to provide improved dielectric materials having low dielectric constant and sufficient mechanical strength and a method and mixture for making same. Due to thermal budget concerns, there is an additional need for a low temperature treatment for the production low dielectric constant materials for integrated circuits.
All references cited herein are incorporated herein by reference in their entirety.